1. Field of the Invention
The present invention relates to a projection exposure method and an apparatus to be used for transferring a mask pattern onto a photosensitive substrate in the lithography step in order to produce a device including, for example, semiconductor devices, image pickup devices (for example, CCD), liquid crystal display devices, and thin film magnetic heads. In particular, the present invention relates to a projection exposure apparatus and a method using the liquid immersion method.
2. Description of the Related Art
A projection exposure apparatus is used, for example, when a semiconductor device is produced, in which an image of a pattern on a reticle as a mask is transferred to respective shot areas on a wafer (or a glass plate or the like) coated with a resist as a photosensitive substrate via a projection optical system. A reduction projection type projection exposure apparatus (stepper), which is based on the step-and-repeat system, has been hitherto frequently used as the projection exposure apparatus. However, a projection exposure apparatus, which is based on the step-and-scan system, is also widely used recently to perform the exposure by synchronously scanning the reticle and the wafer.
As for the resolution of the projection optical system provided for the projection exposure apparatus, As the exposure wavelength to be used is shorter, the resolution becomes higher, while as the numerical aperture of the projection optical system is larger, the higher the resolution becomes higher. Therefore, the exposure wavelength, which is used for the projection exposure apparatus, is shortened year by year as the integrated circuit becomes fine and minute, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, has been already practically used as well.
When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed. Conventionally, in the case of the projection exposure apparatus, the surface of the wafer is adjusted to match the image plane of the projection optical system in the auto-focus manner to perform the exposure. However, it is impossible to adjust and match the wafer surface and the image plane with no error at all. Therefore, it is desirable that the depth of focus δ is large so that no influence is exerted on the image formation performance even when any error remains to some extent. In view of the above, for example, the phase shift reticle method, the modified illumination method, and the multilayer resist method have been hitherto suggested in order to substantially increase the depth of focus as well.
As described above, in the case of the conventional projection exposure apparatus, the depth of focus is gradually decreased, as the exposure light beam has the shorter wavelength, and the numerical aperture of the projection optical system is increased. In order to respond to the further higher integration of the semiconductor integrated circuit, the investigation is also made to further shorten the exposure wavelength. If such a situation is continued as it is, then the depth of focus is excessively decreased, and it is feared that the margin may be insufficient during the exposure operation.
Accordingly, the liquid immersion method has been suggested as a method for substantially shorten the exposure wavelength and increase the depth of focus. In this method, the space between the lower surface of the projection optical system and the wafer surface is filled with a liquid such as pure water or any organic solvent so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases). A technique, which is described, for example, in International Publication No. 99/49504, is exemplified as a conventional technique concerning the projection exposure apparatus and the exposure method to which the liquid immersion method is applied.
In the liquid immersion method as described above, for example, the pure water or the organic solvent is used as the liquid with which the space between the lower surface of the projection optical system and the wafer surface is filled. Any one of the liquids, which is used in this method, has the high electric insulation. For example, the ultrapure water, which is used in the semiconductor production factory, has a specific resistance of about 15 MΩ·cm which is high. The liquid, which has the high insulation as described above, tends to be charged with the static electricity due to the friction with the piping and/or the cavitation generated in the orifice provided in the piping passage when the liquid is made to flow through the piping passage. If the liquid, which is charged with the static electricity, is used for the liquid immersion method, it has been feared that the electric discharge may be caused between the liquid and the circuit pattern having been already formed on the wafer, and the circuit pattern may be destroyed. Further, if the electric discharge is caused between the liquid and any object other than the circuit pattern, it has been feared that the electric equipment, which is arranged around the projection optical system or around the wafer, may malfunction due to the electric noise generated during the electric discharge, and the projection exposure apparatus may cause any error and/or the projection exposure apparatus may be stopped. Further, the charged liquid attracts surrounding impurities by the static electricity. Therefore, the impurities may inhibit the exposure in some cases.